Shining light on the electronic structure and relaxation dynamics of the isolated oxyluciferin anion.

Firefly bioluminescence is exploited widely in imaging in the biochemical and biomedical sciences; however, our fundamental understanding of the electronic structure and relaxation processes of the oxyluciferin that emits the light is still rudimentary. Here, we employ photoelectron spectroscopy and quantum chemistry calculations to investigate the electronic structure and relaxation of a series of model oxyluciferin anions. We find that changing the deprotonation site has a dramatic influence on the relaxation pathway following photoexcitation of higher lying electronically excited states. The keto form of the oxyluciferin anion is found to undergo internal conversion to the fluorescent S1 state, whereas we find evidence to suggest that the enol and enolate forms undergo internal conversion to a dipole bound state, possibly via the fluorescent S1 state. Partially resolved vibrational structure points towards the involvement of out-of-plane torsional motions in internal conversion to the dipole bound state, emphasising the combined electronic and structural role that the microenvironment plays in controlling the electronic relaxation pathway in the enzyme.

Photoelectron spectroscopy of isolated luciferin and infraluciferin anions in vacuo: competing photodetachment, photofragmentation and internal conversion.

The electronic structure and excited-state dynamics of the ubiquitous bioluminescent probe luciferin and its furthest red-shifted analogue infraluciferin have been investigated using photoelectron spectroscopy and quantum chemistry calculations. In our electrospray ionization source, the deprotonated anions are formed predominantly in their phenolate forms and are directly relevant to studies of luciferin and infraluciferin as models for their unstable oxyluciferin and oxyinfraluciferin emitters. Following photoexcitation in the range 357-230 nm, we find that internal conversion from high-lying excited states to the S1(1ππ*) state competes efficiently with electron detachment. In infraluciferin, we find that decarboxylation also competes with direct electron detachment and internal conversion. This detailed spectroscopic and computational study defines the electronic structure and electronic relaxation processes of luciferin and infraluciferin and will inform the design of new bioluminescent systems and applications.

Surface hopping investigation of the relaxation dynamics in radical cations.

Ionization processes can lead to the formation of radical cations with population in several ionic states. In this study, we examine the dynamics of three radical cations starting from an excited ionic state using trajectory surface hopping dynamics in combination with multiconfigurational electronic structure methods. The efficiency of relaxation to the ground state is examined in an effort to understand better whether fragmentation of cations is likely to occur directly on excited states or after relaxation to the ground state. The results on cyclohexadiene, hexatriene, and uracil indicate that relaxation to the ground ionic state is very fast in these systems, while fragmentation before relaxation is rare. Ultrafast relaxation is facilitated by the close proximity of electronic states and the presence of two- and three-state conical intersections. Examining the properties of the systems in the Franck-Condon region can give some insight into the subsequent dynamics.

Photoelectron spectrum and dynamics of the uracil cation.

The photoelectron spectrum of uracil and the molecular dynamics of its radical cation are investigated using the multiconfigurational time-dependent Hartree (MCTDH) method. For this aim, the vibronic coupling model Hamiltonian is used including up to ten important a' modes. Moreover, to account for coupling through conical intersections between states of different symmetry in the system, coupling constants of two a″ modes are taken into account. The parameters used in the model are obtained by fitting to ab inito data obtained with extensive EOM-IP-CCSD calculations. The first four cationic states were investigated, which are either of A″ (hole in a π orbital) or A' (hole in a nO orbital) symmetry. The results of the wavepacket propagations were used to calculate the corresponding photoelectron spectrum and compare to the experimental spectrum. The MCTDH simulations reproduce the experimental spectrum well. The dynamics starting from the D2 and D3 ionic states show a fast relaxation to the cationic ground state often involving direct D2-D0 or D3-D1 transitions.

9D nonadiabatic quantum dynamics through a four-state conical intersection: investigating the homolysis of the O-O bond in anthracene-9,10-endoperoxide.

The excited state dynamics of anthracene-9,10-endoperoxide is investigated using quantum wavepacket dynamics. APO is an aromatic endoperoxide which, upon excitation to S(1), shows a cleavage of the oxygen-oxygen bond, leading to rearrangement products. It was shown that the dynamics of the O-O cleavage is modulated by a four-state degeneracy [D. Mollenhauer, I. Corral, and L. González, J. Phys. Chem. Lett. 1, 1036 (2010)]. The most important mode to describe the O-O cleavage is the opening of the O-O bond. Excitation to higher excited states S(n) (n ≥ 2) leads to the release of singlet molecular oxygen. For this release, the twist of the oxygen atoms around the molecular axis is an important mode. These two degrees of freedom were employed to calculate two-dimensional potential energy surfaces for the four singlet states which become degenerate. Further modes were included in terms of harmonic oscillators. Using the multiconfigurational time-dependent Hartree method, quantum dynamic simulations were performed in up to nine degrees of freedom. Moreover, the nine branching space vectors, which prove the degeneracy to be a four-state conical intersection (4CI), were calculated and included in the wavepacket propagations. The resulting dynamics show that the 4CI is reached very fast (in less than 30 fs after excitation) and the wavepacket distributes over all states. The degree of distribution into the states is very much dependent on which modes are included in the simulation.

On the light-driven isomerization of a model asymmetric molecular rotor: conformations and conical intersections of 2-cyclopentylidene-tetrahydrofuran.

The ground state potential energy surface of the model molecular rotor 2-cyclopentylidene-tetrahydrofuran (CPTHF) has been characterized by calculating minimum energy conformations, racemization pathways, and rotational barriers with high level ab initio electronic structure calculations. Two conformers with their corresponding enantiomers are found. The activation barriers for racemization are negligible, therefore thermal racemization takes place at room temperature. Torsional transition states, calculated using multiconfigurational CASSCF calculations, show twisted and pyramidalized biradical structures. Additionally, the photochemistry of CPTHF has been investigated using the accurate MS-CASPT2/CASSCF methodology. In the UV spectrum it is found that the spectroscopic state is the S(1), which corresponds to a pipi* transition within the ethylene moiety. To understand light-triggered isomerization around the C=C bond, five conical intersections between the S(0) and S(1) have been located for each conformer of CPTHF, which allow the system to rapidly decay to the electronic ground state.

Unravelling the Role of an Aqueous Environment on the Electronic Structure and Ionization of Phenol Using Photoelectron Spectroscopy.

Water is the predominant medium for chemistry and biology, yet its role in determining how molecules respond to ultraviolet light is not well understood at the molecular level. Here, we combine gas-phase and liquid-microjet photoelectron spectroscopy to investigate how an aqueous environment influences the electronic structure and relaxation dynamics of phenol, a ubiquitous motif in many biologically relevant chromophores. The vertical ionization energies of electronically excited states are important quantities that govern the rates of charge-transfer reactions, and, in phenol, the vertical ionization energy of the first electronically excited state is found to be lowered by around 0.8 eV in aqueous solution. The initial relaxation dynamics following photoexcitation with ultraviolet light appear to be remarkably similar in the gas-phase and aqueous solution; however, in aqueous solution, we find evidence to suggest that solvated electrons are formed on an ultrafast time scale following photoexcitation just above the conical intersection between the first two excited electronic states.

ortho and para chromophores of green fluorescent protein: controlling electron emission and internal conversion.

Green fluorescent protein (GFP) continues to play an important role in the biological and biochemical sciences as an efficient fluorescent probe and is also known to undergo light-induced redox transformations. Here, we employ photoelectron spectroscopy and quantum chemistry calculations to investigate how the phenoxide moiety controls the competition between electron emission and internal conversion in the isolated GFP chromophore anion, following photoexcitation with ultraviolet light in the range 400-230 nm. We find that moving the phenoxide group from the para position to the ortho position enhances internal conversion back to the ground electronic state but that adding an additional OH group to the para chromophore, at the ortho position, impedes internal conversion. Guided by quantum chemistry calculations, we interpret these observations in terms of torsions around the C-C-C bridge being enhanced by electrostatic repulsions or impeded by the formation of a hydrogen-bonded seven-membered ring. We also find that moving the phenoxide group from the para position to the ortho position reduces the energy required for detachment processes, whereas adding an additional OH group to the para chromophore at the ortho position increases the energy required for detachment processes. These results have potential applications in tuning light-induced redox processes of this biologically and technologically important fluorescent protein.

Substituent effects on the light-induced C-C and C-Br bond activation in (bisphosphine)(eta2-tolane)Pt0 complexes. A TD-DFT study.

A theoretical study of the steric and electronic effects of different substituents on the electronic ground and excited states of (bisphosphine)(eta2-tolane)Pt0 complexes is presented. A natural-bond-order (NBO) analysis has been performed to describe the bonding nature of the "hapto-like" coordination of the Pt atom to the alkyne bond of the tolane group. The results show an important contribution of the pi-back donation in all complexes, amounting to half of the energy associated to the sigma-bonding interaction. A TD-DFT study of the absorption spectra helps rationalizing the photochemistry of the complexes. Metal-ligand charge transfer (MLCT) transitions from the Pt atom to the alkyne are assigned as the photochemical "active" states responsible for C-C bond cleavage. Electronic excitations to the sigma* orbital of the C-Br bond are involved in C-Br bond activation. It is shown that both steric and electronic effects play an important role in determining the presence of these electronic excitations.